Steam keeps reporting new records. After Skyrim became the fastest selling game in the platform's history, it has now helped push Steam past the 5 million concurrent users mark:

"Valve's Steam and Game Stats page shows, at the time of publication, over the past 24 hours a peak of 5,012,468 users were logged in. Skyrim remains the most-played game on Steam by a huge margin - double the amount playing Team Fortress 2, the second-placed game."